"When they surprised a grizzly sow with cubs. In an apparent attempt to defend a perceived threat to her cubs, the bear attacked and fatally wounded the man."

The AP adds that Yellowstone spokesman Al Nash "said the couple saw the bear twice on their hike. The first time, they continued hiking. The second time, the grizzly charged them and the man told his wife to run."

The woman survived and other hikers heard her calls for help and dialed 911, park officials say.

Park rangers are looking for the bear. They're also clearing the area "of all backcountry users," officials say. It isn't clear yet whether the couple were carrying pepper spray that has proved useful at fending off such attacks.

The couple's names have not yet been released.

Montana's KTVM-TV says the news that the bear hasn't been located is making some park visitors nervous.

The Bozeman Daily Chronicle notes that while "a bear warning sign was posted at the Wapiti Lake Trailhead ... there hadn't been any reports of bear encounters on or along the trail this season. There had also been no recent reports of animal carcasses on or along the trail."

"Matayoshi and his wife Marylyn were hiking Wednesday morning on the Wapiti Lake Trail, which is located off the South Rim Drive, south of Canyon Village and east of the park's Grand Loop Road.

"The couple was hiking west back toward their vehicle. At approximately 11:00 a.m., at a point about a mile and a half from the trailhead, they walked out of a forested area into an open meadow. It appears that the couple spotted a bear approximately 100 yards away and then began walking away from the bear. When they turned around to look, they reportedly saw the female grizzly running down the trail at them. The couple began running, but the bear caught up with them, attacking Mr. Matayoshi. The bear then went over to Mrs. Matayoshi, who had fallen to the ground nearby. The bear bit her daypack, lifting her from the ground and then dropping her. She remained still and the bear left the area."

Update at 3:25 p.m. ET. Bear Will Not Be Captured:

The Associated Press now writes that "Yellowstone National Park authorities will not try to capture a female grizzly that killed a backcountry hiker because it was trying to defend its cubs when it was surprised by the man, a spokesman said Thursday. Park spokesman Al Nash said Wednesday's mauling of the 57-year-old man was a purely defensive act. He said Yellowstone typically does not try to capture or remove a bear in what he called 'a wildlife incident.' "

Update at 11:35 a.m. ET. More background, from the AP:

"Yellowstone and surrounding areas are home at least 600 grizzlies — and some say more than 1,000. Once rare to behold, grizzlies have become an almost routine cause of curious tourists lining up at Yellowstone's roadsides at the height of summer season.

"Those tourists have been flooding into Yellowstone in record numbers: 3.6 million last year, up 10 percent from 2009's 3.3 million, also a record. In June 2010, a grizzly just released after being tranquilized for study killed an Illinois man hiking outside Yellowstone's east gate. Last July, a grizzly killed a Michigan man and injured two others in a nighttime campground rampage near Cooke City, Montana, northeast of the park."

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